Insulated gate field effect transistor (IGFET) devices are widely used in modern electronic applications. Metal-oxide-semiconductor field effect transistor (MOSFET) devices and lateral-(double)-diffused-metal-oxide-semiconductor (LDMOS) devices are well known examples of such IGFET devices. The term metal-oxide-semiconductor and the abbreviation MOS are to be interpreted broadly. In particular, it should be understood that they are not limited merely to structures that use “metal” and “oxide”, but may employ any type of conductor, including “metal”, and any type of dielectric, including “oxide”. The term field effect transistor is abbreviated as “FET”. It is known that improved performance of LDMOST devices can be obtained by using reduced surface field (RESURF) structures.
Power transistor devices are designed to be tolerant of the high currents and voltages that are present in power applications such as motion control, air bag deployment, and automotive fuel injector drivers. One type of power MOS transistor is a laterally diffused metal-oxide-semiconductor (LDMOS) transistor. In an LDMOS device, a drift space is provided between the channel region and the drain region.
LDMOS devices may be designed to operate in a high-side configuration in which all of the device terminals are level shifted with respect to the substrate potential. Devices configured for high-side operation have been applied in power switchers in DC-to-DC converters, which have respective LDMOS devices for the high side and low side. High-side capable devices are designed to prevent a direct forward bias or punch-through path from a body region of the LDMOS device to an underlying substrate.
LDMOS devices are often used in applications, such as automotive applications, involving operational voltages greater than 40 volts. Breakdown resulting from applying such high voltages to the drain is often prevented through a reduced surface field (RESURF) structure in the LDMOS device design. The RESURF structure is designed to deplete the drift space of the LDMOS device in both vertical and lateral directions, thereby reducing the electric field near the surface at the drift region and thus improving the off-state breakdown voltage (BVdss) of the device.
Some LDMOS devices have a “double RESURF” structure. For example, in n-channel LDMOS devices, the drift space contains an upper level n-type region and a lower level p-type region, with an n-type buried isolation layer beneath the p-type region. The double nature of the structure refers to the depletion of the two regions and the reduction of the electric field in the related junction areas. Double RESURF structures typically apply the drain voltage to isolation regions in order to deplete the both the n-type and p-type regions.
However, biasing the isolation regions at the drain voltage increases the field stress between the body of the LDMOS device and a buried isolation layer. Breakdown may instead occur between the body and the buried isolation layer, thereby limiting the breakdown voltage. Previous efforts to address such source/body-based breakdown have introduced fabrication challenges or degraded the electrostatic discharge (ESD) and safe operating area (SOA) performance of the device. Improvements have been devised to achieve high breakdown voltage and good SOA performance. Despite improvements, the device has a relative high current gain in the parasitic bipolar transistor formed by the p-type device body, n-type isolation ring, and p-type substrate. As a result, a large substrate current has been detected during the circuit switch while the inductive component in the circuit brings a negative potential on the source/body terminal.